33
Fundamentals of Electric Capacitors
a desired radio station frequency. These tuner capacitors have low capacitance
values in a range of 10 to 500 pF and are used primarily for frequency filtering.
The dielectric materials used in these devices include air, plastic foil, and inert
sulfur hexafluoride gas. A typical variable device is a trimmer capacitor, which
is a miniaturized version of a variable capacitor. In variable capacitors, several
dielectric materials including air, ceramic, mica, and polymers are used.
1.7.3 Power Capacitors
Power capacitors are generally large and heavy in comparison to fixed capacitors. A typical weight range is 10 to 50 kg. Smaller power capacitors are also
available and have capacitance ranges from 2.5 to 20 μF with an AC circuit
working voltage of 150 V and an RMS voltage as high as 550 V. These devices
are normally used in electrical engines as starting motor capacitors.
The dielectric materials used in power capacitors include paper, polypropylene, and mixed vacuums impregnated with mineral or synthetic oil. The
most reliable dielectric material is polypropylene. The metal electrodes can
be aluminum foils or thin metal films deposited onto the surfaces of the
dielectrics. The reliabilities and lifetimes of power capacitors are always concerns because of the possibility of overheating.
1.7.4 High-Voltage Capacitors
For DC voltage or low frequency applications, polymer foil-based capacitors
are used. Ceramic capacitors are more suitable for high frequency applications. Capacitors shaped like disks and tubes have been fabricated. The tubeshaped types are easier to protect against hazardous flashovers and can be
used with voltage ratings up to 6 kV. A series connection of several individual elementary capacitors with matching capacitances is a way to reduce the
risk of high voltage when they are used on some AC circuits.
When using high-voltage capacitors, it is important to be aware of dielectric absorption (the percentage ratio of the regained potential with respect
to the charging potential). For purposes of accident prevention, high-voltage
capacitors are required to be shunted to high-rated resistors when they are
switched from the main intended circuit.
1.7.5 Interference-Suppression Capacitors
In general, interference-suppression capacitors have low inductances. These
capacitors can be separated into two classes depending on their use: Class X
and Class Y. Class X types are used in applications with lower safety requirements, for example, where a collapse would not pose a danger. Class Y types
are used in environments or applications that have strict safety requirements.
These two classes have different test conditions. For example, an 1100 V test
voltage is used for testing Class X capacitors such as a 4 μF/250 V capacitor; a
Fundamentals of Electric Capacitors
a desired radio station frequency. These tuner capacitors have low capacitance
values in a range of 10 to 500 pF and are used primarily for frequency filtering.
The dielectric materials used in these devices include air, plastic foil, and inert
sulfur hexafluoride gas. A typical variable device is a trimmer capacitor, which
is a miniaturized version of a variable capacitor. In variable capacitors, several
dielectric materials including air, ceramic, mica, and polymers are used.
1.7.3 Power Capacitors
Power capacitors are generally large and heavy in comparison to fixed capacitors. A typical weight range is 10 to 50 kg. Smaller power capacitors are also
available and have capacitance ranges from 2.5 to 20 μF with an AC circuit
working voltage of 150 V and an RMS voltage as high as 550 V. These devices
are normally used in electrical engines as starting motor capacitors.
The dielectric materials used in power capacitors include paper, polypropylene, and mixed vacuums impregnated with mineral or synthetic oil. The
most reliable dielectric material is polypropylene. The metal electrodes can
be aluminum foils or thin metal films deposited onto the surfaces of the
dielectrics. The reliabilities and lifetimes of power capacitors are always concerns because of the possibility of overheating.
1.7.4 High-Voltage Capacitors
For DC voltage or low frequency applications, polymer foil-based capacitors
are used. Ceramic capacitors are more suitable for high frequency applications. Capacitors shaped like disks and tubes have been fabricated. The tubeshaped types are easier to protect against hazardous flashovers and can be
used with voltage ratings up to 6 kV. A series connection of several individual elementary capacitors with matching capacitances is a way to reduce the
risk of high voltage when they are used on some AC circuits.
When using high-voltage capacitors, it is important to be aware of dielectric absorption (the percentage ratio of the regained potential with respect
to the charging potential). For purposes of accident prevention, high-voltage
capacitors are required to be shunted to high-rated resistors when they are
switched from the main intended circuit.
1.7.5 Interference-Suppression Capacitors
In general, interference-suppression capacitors have low inductances. These
capacitors can be separated into two classes depending on their use: Class X
and Class Y. Class X types are used in applications with lower safety requirements, for example, where a collapse would not pose a danger. Class Y types
are used in environments or applications that have strict safety requirements.
These two classes have different test conditions. For example, an 1100 V test
voltage is used for testing Class X capacitors such as a 4 μF/250 V capacitor; a